1. Field of the Invention
The invention relates to a shadow projection mask for ion implantation and ion beam lithography. The mask utilizes a thin P-doped silicon layer with through holes adapted to the mask pattern together with a grid supporting the layer in regions without holes and being composed of silicon ribs doped differently from the silicon layer. The layer on the side facing away from the grid is covered with an ion-absorbing layer, and at least the surface exposed to ion radiation is electrically and thermally conductive.
2. Prior Art
Ion implantation is a well known technique that is widely used for doping semiconductor material. For selectively doping a semiconductor substrate, e.g. a silicon wafer, the generally applied method is to generate a mask on the semiconductor surface. Accordingly, on the surface, a layer of one material, e.g. a photoresist or a dielectric material, is applied and subsequently a pattern with through holes not covering the semiconductor regions to be doped is generated in the layer. The disadvantage of such masks is that they can only be used for the implantation process, and furthermore, they often have to be subsequently removed.
According to another known ion implantation method not requiring such masks, an ion beam whose diameter is smaller than the dimension of the smallest doped region to be made, moves over the semiconductor body in such a manner that at the end of the irradiation process each point on the semiconductor surface has been in the beam path. The ion beam during this process is blanked out under computer control in all those spots where the semiconductor material is not to be irradiated. With this method, doped semiconductor structures comprising very complex and minuscule structural elements can be made with satisfactory precision. The disadvantage of this method, however, is that it involves a very complex apparatus, and the attainable throughput will only suffice for series production if a very high ion current is used.
In European patent application No. 00 01 038, a silicon mask is described which can also be used for ion implantation. This mask is self-supporting and consists of a thin silicon layer with through-openings. A frame, also made of silicon but of lower doping then the silicon layer, supports the silicon layer in the regions without mask openings. This mask is placed onto the semiconductor substrate to be doped prior to ion implantation, or onto a .ltoreq.20 .mu.m thick spacer ring, respectively on the semiconductor substrate, and is removed again after ion implantation. The mask can thus be repeatedly used. Experience has shown, however, that after frequent use of the mask the thin silicon layer partially loses its mechanical stability, and consequently, the silicon mask irreversibly changes its shape. This is unacceptable if very small and closely packed doped regions are to be made with the mask.
With increasing microminiaturization in the semiconductor field, through which pattern elements to be transferred lithographically are getting progressively smaller, electron beam lithography will soon have reached its technical limits. Therefore lithography by means of ion beams, which compared with electron beams, exhibit some considerable advantages and is becoming more interesting. For example, unlike electron beams, ion beams have a negligible proximity effect. The advantages of ion beams, vis-a-vis electron beams, are discussed in R. L. Seliger and P. A. Sullivan, "Ion Beams Promise Practical Systems for Submicrometer Wafer Lithography", Electronics, Mar. 27, 1980, p. 142 et seq. This article also reveals that in ion lithography, similarly to ion implantation, the selectivity of application can be accomplished either through a deflectable focussed beam, or through the use of masks. The masks described therein consist of an ionabsorbing material placed onto a supporting film of monocrystalline silicon, or of aluminum oxide. Hence, with these masks, the ions used for irradiation have to penetrate a material before reaching the substrate to be irradiated.
German Offenlegungsschrift No. 29 22 416 describes masks which are preferably used in electron beam lithography, but also find application in ion beam lithography. In the regions which are not covered by the mask material, the masks described in the Offenlegungsschrift have throughgoing holes in the supporting film which consists of monocrystalline P.sup.+ -doped silicon and suspended between the grid ribs. Through holes in the supporting layer are advantageous because ions which have to penetrate a layer, be it ever so thin, are necessarily scattered. While there is much less scattering than if electrons are used, but for transferring structures of .ltoreq.0.5 .mu.m even the use of a socalled "channelling layer" is no longer acceptable. A "channelling layer" is a monocrystal membrane through which with an optimum angle of bombardment the ions can penetrate with only a minimum scattering.
Originally it had been assumed that for ion beam and electron beam lithography basically the same masks can be used. As specified, however, by U. Behringer and R. Speidel during "Microcircuit Engineering 81" congress in Lausanne from Sept. 28 to 30, 1981, in a lecture entitled "Investigation of the Radiation Loads of a Self-Supporting Silicon Mask in an Ion Beam Proximity Printer", irradiation with ions can cause a reversible or irreversible modification of the mask. In their tests, Behringer and Speidel used three types of masks which all have a basic frame of a film with physical holes, which consisted of P.sup.+ -doped monocrystalline silicone. The film was supported by a grid of silicon ribs. The masks of a first type were coated on both sides with gold, the masks of a second type were coated on both sides with aluminum, and the masks of a third type consisted only of the silicon frame.
The masks of the first type did not reveal to the authors any important changes (&gt;0.2 .mu.m). The masks of the second type became gradually non-planar through ion bombardment. However, the non-planarity could be eliminated again by means of a tempering for 15 minutes at 400.degree. C. The masks of the third type were destroyed after a relatively short period of use, which in the opinion of the authors was due to the poor thermal conductivity of the mask. Although the results attained by the authors indicate that the masks of the first type are suitable for ion beam lithography, the inventors of the subject of the present application have found that when gold-coated ion beam masks are used there is contamination of the irradiated substrates with gold. This is unacceptable, particularly because of the strong effects of gold in semiconductor material.